1. Field of the Invention
The present invention relates to an adhesive composition that is halogen-free and yields a cured product, on curing, that exhibits excellent flame retardancy, and also relates to an adhesive sheet, a coverlay film, and a flexible copper-clad laminate that use such a composition.
2. Description of the Prior Art
Conventionally, the adhesives used in electronic materials such as semiconductor sealing materials and glass epoxy-based copper-clad laminates have included a bromine-containing epoxy resin or phenoxy resin or the like in order to ensure a superior level of flame retardancy. However, because compounds containing halogens such as bromine release toxic gases such as dioxin-based compounds when combusted, in recent years, the use of halogen-free materials in adhesives has been keenly investigated.
On the other hand, flexible copper-clad laminates, which are thinner than the glass epoxy-based copper-clad laminates mentioned above, and offer additional flexibility, are now being widely used, and the size of this market continues to expand as electronic materials become thinner, and move to even higher density packaging. Flexible copper-clad laminates are copper-clad laminates with improved flexibility, which are produced by bonding a polyimide film and a copper foil together with an adhesive, and then heating and curing the adhesive. In a similar manner to the adhesives used in the electronic materials described above, the use of halogen-free materials in the adhesives used in these flexible copper-clad laminates is also being investigated.
Furthermore, once the copper foil of a flexible copper-clad laminate has been processed to form a wiring pattern, an electrically insulating film (a coverlay film) such as a polyimide film coated with an adhesive film is used to cover the surface on which the wiring pattern has been formed, thereby protecting the wiring. The properties required for the materials for these flexible copper-clad laminates and coverlay films include favorable adhesion between the electrically insulating film and the copper foil, as well as favorable heat resistance, solvent resistance, electrical characteristics (anti-migration properties), dimensional stability, storage stability, and flame retardancy. In addition, when flexible printed wiring boards prepared by crimping a coverlay film are bonded together to form multilayered structures with increased density, the adhesive films (adhesive sheets) used for bonding the boards together require the same characteristics as those required by flexible copper-clad laminates and coverlay films.
Examples of known materials that satisfy the above requirements include adhesive compositions comprising an epoxy resin, an aromatic phosphate ester, a curing agent, and a high-purity acrylonitrile butadiene rubber, as well as flexible copper-clad laminates and coverlays that use such adhesive compositions (see patent reference 1), but high-purity acrylonitrile butadiene rubber is extremely expensive, meaning that with the exception of certain special applications, large-scale use of this material is impossible. In addition, adhesive compositions comprising an epoxy resin, an aromatic phosphate ester, a nitrogen-containing phenol novolac resin, and a normal purity acrylonitrile butadiene rubber, as well as flexible copper-clad laminates and coverlays that use such adhesive compositions, are also known (see patent reference 2), but because these materials use normal purity acrylonitrile butadiene rubber, the anti-migration properties tend to deteriorate.
[Patent Reference 1] JP 2001-339131A
[Patent Reference 2] JP 2001-339132A